LX 255 Trouble - Need Help

Hope someone can help me. I have a LX 255 lawn tractor which has been trouble free for ~ 9 years. I have done the regular basic maintenance on the tractor and up until last week it ran very well. Last week after about 15 minutes of mowing the tractor just shut off. Starter would crank fine and it was getting gas (pulled the plug) but no spark / ignition. Didn't matter if the parking brake was on or off or I was in the seat or not ... just would not re start. The following day just for the heck of it I turned the key and it started right up. Went to use it today (knowing that the problem might return) and the mower started fine, however, as soon as I released the parking brake it would shut off even though I was in the seat. I took the seat switch out and pushed the seat switch in and out a few times and re installed. Seemed to fix the problem and I began to mow my lawn. After about 20 minutes same problem ... mower just shut off and will not start. Again it is getting gas and starter cranks fine but no ignition. Could a problem like this be caused by a faulty seat switch or do I have a bigger problem in the ignition or safety interlock system? BTW the LX 255 uses a air cooled single cylinder 15hp koehler engine. Simple and reliable (until now). Note - just tested mower again this morning and it starts right up. Any ideas on what is causing this problem? There is no overheating type kill switch on the mower is there?